The present invention relates to a method for forming a gate electrode or word line in a highly integrated and high speed semiconductor device such as 1 giga bit or more DRAM.
Presently, the gate electrode of a semiconductor device is typically composed of a doped polysilicon only or a polycide including a doped polysilicon film and a tungsten silicide (WSi.sub.x) sequentially deposited for reducing a resistivity thereof. However, the resistivity of tungsten silicide film is about 100 .mu..OMEGA..cndot.cm, so that it is still too high to be used in a 1 giga bit or more DRAM that generally requires a submicron technology (e.g. 0.18 .mu.m or below linewidth fabrication technology). For example, in a MOS device, when the delay of an interconnect path is bigger than that of gate switching the device malfunctions. Accordingly, for the formation of a highly integrated and high performance of device, the resistivity of interconnect path (e.g. the interconnect path between the word line and the bit line in a DRAM) should be reduced as lower as possible. Therefore, the study related to a tungsten gate electrode (or word line) of 10 .mu..OMEGA..cndot.cm in resistivity has been continuously made.
In conventional methods of the formation of a tungsten gate electrode, a tungsten film or a polysilicon/tungsten film is deposited on a gate oxide and then selectively etched, using a gate electrode photoresist pattern to form a gate electrode. However, this conventional method has a disadvantage that the patterning for the gate electrode gives an attack to the gate oxide. Though the damage of the gate oxide can be recovered by re-oxidation process that is made for preventing the deterioration of the gate oxide and protecting the underlying semiconductor substrate during an ion implantation of LDD source/drain formation to be performed later, it causes another problems. That is, the re-oxidation makes a tungsten oxide film on the surface of gate electrode so that the overall resistivity of the gate electrode is rather increased and it destroys the gate (or word line) profile.
Also the boundary of the tungsten film and the gate oxide is so unstable that a separate glue layer may be required between the tungsten film and the gate oxide. However, since the glue layer is generally composed of the high resistivity of materials, the overall resistivity of the gate electrode is increased.
In addition, when the tungsten electrode is deposited by CVD (chemical vapor deposition) the fluorine element of source gas WF.sub.6 attacks the gate oxide.